There is provided a related-art audio apparatus which is equipped with a D/A (Digital/Analog) converter and an audio amplifier. In a volume adjusting circuit included in the audio apparatus, gain adjusting of an audio signal is performed using one or both of a digital circuit and an analog circuit. A digital gain variable circuit which performs gain adjusting by the digital circuit is effective in reducing a circuit area. However, digital clipping could occur with a positive gain setting. Thus, a technique is known of adding an analog gain variable circuit to perform gain adjusting in order to obtain a sufficiently large analog output amplitude while preventing the digital clipping.
FIG. 1 is a diagram which explains the volume adjusting circuit included in the related-art audio apparatus. A related-art volume adjusting circuit 10 includes a D/A converter 1; a digital gain variable circuit 2 which is connected to a former stage of the D/A converter 1; and an analog gain variable circuit 3 which is connected to a latter stage of the D/A converter 1.
Gain adjusting is performed by two independent registers, a digital gain setting register 5 and an analog gain setting register 7, setting a digital gain and an analog gain. The digital gain variable circuit 2 is controlled by a digital gain control unit 4 in accordance with a value set in the digital gain setting register 5. The analog gain variable circuit 3 is controlled by an analog gain control unit 6 in accordance with a value set in the analog gain setting register 7.
FIG. 2 is a diagram which explains an example of related-art variable gain control. A digital gain setting value G1 is a value set to the digital gain setting register 5, while a digital gain DG is a value to be set to the digital gain variable circuit 2. An analog gain setting value G2 is a value set to the analog gain setting register 7, while an analog gain AG is a value to be set to the analog gain variable circuit 3. The digital gain DG and the analog gain AG are independently controlled by the digital gain setting register 5 and the analog gain setting register 7.
Moreover, Patent document 1 discloses, for example, having a digital variable gain unit and an analog variable gain unit, wherein only the digital variable gain unit is controlled for a setting greater than 0 dB and only the analog variable gain unit is controlled or both the analog and digital variable gain units are controlled for a setting less than 0 dB. Furthermore, Patent document 2 discloses receiving a signal such as digital data and an image from an analog transmission line to demodulate the received signal.